The present invention relates to a method and apparatus for utilizing biofuel or waste material in energy production.
Many methods have been suggested for utilizing cheap biofuel or waste material for energy production in order to compensate for a portion of fossil fuel used in energy production and to thereby decrease CO.sub.2 emissions generally in energy production. In most cases, waste material has to be disposed of in one way or another anyhow.
Biofuel, and especially waste material, being a low energy fuel and often including harmful and noxious components, is not easily utilized for energy production. Noxious components, such as heavy metal compounds, may cause environmentally dangerous emissions, whereas alkali compounds tend to cause technical problems, especially when burning such material.
The low energy content of biofuel and waste material render them generally inadequate for high efficiency production of energy, such as high temperature, high pressure steam or electricity. Also, uneven delivery of waste material, differences in its quality and other similar hard-to-control variations, may cause problems in energy production that relies solely on waste-material-based fuel. The waste material per se is often delivered in a state unsuitable for direct introduction into a steam boiler. The material has to be dried, crushed, pelletized or otherwise handled before it is fed into the boiler. Therefore there have been suggested different ways of combining gasifiers with combustors.
Biofuel and waste material can, however, rather easily be gasified. But the utilization of biofuel and waste material for the production of energy presupposes a complete gasification plant, with gasifier and gas cleaners, and a complete power plant, with boiler, turbines, etc. Such is not economically feasible to provide when only small amounts of biofuel or waste material are available.
It has been suggested in Finnish patent application FI 1895/73 to gasify biofuel on a grid and introduce the gas produced in the gasifier into a coal or oil fired boiler to be burned therein. The gas is introduced into the boiler together with the oil or coal through special oil or coal burners.
It has been suggested in another Finnish patent application FI 851338 that waste wood or similar low energy fuel be gasified and the gas thus produced be burned in a multi-fuel combustor. Additional pulverized combustible material is injected directly into the gas burning flame.
It has further been suggested in WO publication 90/12986 to dry waste material in direct heat exchange with hot exhaust gases from a coal-burning plant and feeding both the gas mixture produced through drying and the dry solid material produced from the waste into the coal burning plant. The gas is introduced into the bottom of the coal burning reactor below coal burners therein. Dried material is also fed into the reactor at a higher level.
It has also already been suggested in DE 36 32 534 to gasify fuel which contains alkali compounds in a fluidized bed gasifier at a temperature below 900.degree. C. (1652.degree. F.) and to burn the gas produced in the gasifier in a combustor at a temperature level high enough (&gt;900.degree. C.) to melt alkali contents in the flue gas. The alkali compounds can be separated in a molten state, and the cleaned gas can be cooled in a convection section of a boiler.
All the above suggested processes seem to presuppose that the combustor, boiler or burners therein are constructed especially for that special process, for continuously burning biofuel or waste derived product gas. Some of the designs seem to be rather complicated.